Tunable RF devices such as filters, phase shifters, and oscillators are typically built using semiconductor diodes, so called varactors, in which the capacitance is controlled via external bias. While the main line varactors are inexpensive and robust, they are only suitable for applications up to 10 GHz. Above this frequency, the energy dissipated in such varactors is prohibitively high (low quality factor Q). In some GaAs varactors, the range of operation is extended to much higher frequencies. The high cost of manufacturing for such devices, however, makes them impractical for most applications.
Recently, tunable dielectrics, such as Balium Strontium Titanate (BST), have been employed as the active elements in tunable capacitor devices and are becoming increasingly important for a large number of microwave applications. Utilizing a tunable dielectric element in tunable capacitance devices, especially at frequencies over 20 GHz, has been shown to increase the figure of merit (performance vs. noise) of the tunable capacitor device with a lower cost than other conventional technologies. BST thin film and especially BST/MgO thick and thin films composites have demonstrated unparalleled performance at high MW frequencies up to 60 GHz. They also have low power requirements, but need voltages in some applications. Thus, incorporation of the tunable dielectric elements provides high performance at low cost.
While the figure of merit of the tunable dielectric devices can be sufficiently high, such as those with composite materials, the voltage requirements of these devices are typically too high (300V). The standard employed for the lower frequency applications typically designs for tuning voltages in the range of 20–40 V. There is a pressing need to develop lower voltage tunable devices with a high figure of merit so as to achieve high levels of performance at microwave frequencies, i.e., this requires the amount of tuning to be maximized and the amount of loss to be minimized, while satisfying industry requirements for the low operating voltages.